Method of manufacturing aluminum alloy and aluminum alloy

ABSTRACT

A method of manufacturing aluminum alloy may include solutionizing a cast product made of an aluminum alloy containing 0.1 to 0.3 wt % of Zr; quenching the cast product at a temperature of 30° C. or less after the solutionizing; and aging the cast product after the quenching.

CROSS-REFERENCE(S) TO RELATED APPLICATIONS

This application claims priority to Korean Patent Application No. 10-2019-0172978, filed on Dec. 23, 2019 which is incorporated herein by reference in its entirety.

BACKGROUND Technical Field

Exemplary embodiments of the present disclosure relate to a method of manufacturing aluminum alloy and an aluminum alloy produced thereby; and, particularly, to an aluminum alloy for high-power engine components.

Description of Related Art

High-performance vehicles may greatly appeal to consumers seeking driving pleasure, and the development and production thereof are also effective in demonstrating the technology of automobile manufacturers to ordinary consumers.

Such a high-performance vehicle inevitably requires a high-power engine. However, as the power of the engine increases, the physical and thermal loads on the material of the engine increase.

An alloy used to cast a conventional cylinder head for high-performance vehicles has the following compositions, shown in Table 1, and the cylinder head is manufactured by heat treatment as illustrated in FIG. 1.

TABLE 1 Composition Cu (wt %) Si (wt %) Mg (wt %) Fe (wt %) Mn (wt %) Ti (wt %) Al AC4CH 0.2 or less 6.5~7.5 0.35~0.45 0.2 or less 0.03~0.1 0.05~0.2 REM.

The main reinforcing elements of the alloy are Mg and Si. Si is an element that affects the castability and strength of the alloy, and the alloy is improved in strength through the formation of Mg₂Si precipitation phase after heat treatment by Mg.

In other words, the alloy is solutionized to evenly dissolve Si and Mg elements in an Al matrix and aged to form an Mg₂Si compound, resulting in an increase in strength.

Although the aluminum for the cylinder head of the conventional gasoline engine has an endurance limit temperature of about 200° C., the test of the high-power engine shows that the temperature of the cylinder head increases from 250° C. to 300° C. For this reason, the use of existing materials may cause insurmountable damage to the cylinder head.

That is, the high-power engine is damaged before 1/10 of expected endurance test time due to exposure to high-temperature environment during testing. Hence, it can be seen that the conventional alloy compositions do not withstand the harsh environment of the high-power engine.

The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art.

SUMMARY

An embodiment of the present disclosure is directed to an aluminum alloy that can be used in a high-power engine by having excellent high-temperature physical properties and high thermal conductivity in favor of performance and fuel efficiency, and a method of manufacturing the same.

Other objects and advantages of the present disclosure can be understood by the following description, and become apparent with reference to the embodiments of the present disclosure. Also, it is obvious to those skilled in the art to which the present disclosure pertains that the objects and advantages of the present disclosure can be realized by the means as claimed and combinations thereof.

In accordance with an embodiment of the present disclosure, there is provided a method of manufacturing aluminum alloy, which includes solutionizing a cast product made of an aluminum alloy containing 0.1 to 0.3 wt % of Zr, quenching the cast product at a temperature of 30° C. or less after the solutionizing, and aging the cast product after the quenching.

The solutionizing may be performed at a temperature ranging from 520° C. to 560° C. for 4 to 48 hours.

The aging may include primary aging and secondary aging after the primary aging so as to be performed twice.

The primary aging may be performed at a temperature ranging from 165° C. to 195° C. for 4 to 48 hours.

The secondary aging may be performed at a temperature ranging from 200° C. to 225° C. for 3 to 7 hours.

In accordance with another embodiment of the present disclosure, there is provided a method of manufacturing aluminum alloy, which includes solutionizing a cast product made of an aluminum alloy containing 0.1 to 0.3 wt % of Zr, quenching the cast product after the solutionizing, and aging the cast product after the quenching, wherein the aging includes primary aging and secondary aging after the primary aging so as to be performed twice.

The primary aging may be performed at a temperature ranging from 165° C. to 195° C. for 4 to 48 hours.

The secondary aging may be performed at a temperature ranging from 200° C. to 225° C. for 3 to 7 hours.

In accordance with a further embodiment of the present disclosure, there is provided an aluminum alloy that includes Al as a base material, 6.5 to 7.5 wt % of Si, 0.35 to 0.45 wt % of Mg, and 0.1 to 0.3 wt % of Zr.

The aluminum alloy may further include 0.2 wt % or less of Cu.

An Al₃Zr precipitation-strengthening phase may be formed in the aluminum alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a heat treatment process of a conventional alloy.

FIG. 2 illustrates a heat treatment process of an alloy for cylinder heads according to an embodiment of the present disclosure.

FIG. 3 illustrates that an acicular Zr crystal appears in the alloy of the present disclosure.

FIG. 4 illustrates a result of measurement of cylinder head residual stress when an alloy is conventionally quenched.

FIGS. 5A and 5B illustrate a crystallization phase when the alloy is quenched at a temperature of 80° C. and a crystallization phase when the alloy is quenched at a temperature of 30° C., respectively, in the present disclosure.

FIGS. 6A and 6B illustrate a thermodynamic simulation result according to the addition Zr.

FIG. 7 illustrates a precipitation phase when heat treatment is performed on the alloy of the present disclosure.

DETAILED DESCRIPTION

The accompanying drawings for illustrating exemplary embodiments of the present disclosure should be referred to in order to gain a sufficient understanding of the present disclosure, the merits thereof, and the objectives accomplished by the implementation of the present disclosure.

In the exemplary embodiments of the present disclosure, techniques well known in the art or repeated descriptions may be reduced or omitted to avoid obscuring appreciation of the disclosure by a person of ordinary skill in the art.

FIG. 2 illustrates a heat treatment process of an alloy for cylinder heads according to an embodiment of the present disclosure.

A method of manufacturing aluminum alloy and an aluminum alloy produced thereby according to exemplary embodiments of the present disclosure will be described below with reference to FIG. 2.

The present disclosure relates to a composition and manufacturing method of an alloy for cylinder heads capable of realizing high-temperature physical properties and high thermal conductivity to withstand the physical and thermal loads of a high-power engine.

The comparison between the composition of the aluminum alloy according to the present disclosure and the composition of the alloy according to the related art is indicated in the following table, and the aluminum alloy is prepared by T6 heat treatment illustrated in FIG. 2, unlike that illustrated in FIG. 1.

TABLE 2 Composition Cu Si Mg Zr (wt %) (wt %) (wt %) (wt %) Al Related Art 0.2 or less 6.5~7.5 0.35~0.45 — REM. (AC4CH) Present 0.2 or less 6.5~7.5 0.35~0.45 0.1~0.3 REM. Disclosure (AC4CH—Zr)

The aluminum alloy according to the present disclosure contains 0.1 to 0.3 wt % of Zr.

When the Zr content of the aluminum alloy exceeds 0.3 wt %, a coarse acicular Zr-related crystallization phase begins to appear, which is adversely affects the physical properties of the alloy. Therefore, the Zr content is limited to 0.1 to 0.3 wt %.

The aluminum alloy may further include Sr, Mn, Ti, and the like.

The aluminum alloy of the present disclosure has the above compositions shown in Table 2, and a cylinder head cast from the alloy is heat-treated to secure physical properties. In particular, examples of the product cast from the alloy may include not only a cylinder head that requires high-temperature physical properties, but also a component that requires similar properties.

The heat treatment includes solutionizing, quenching, primary aging, and secondary aging.

The solutionizing is performed at a temperature ranging from 520° C. to 560° C. for 4 to 48 hours. As a preferable example, the drawing illustrates that the solutionizing is performed at a temperature of 535° C. for 6 hours.

Unlike the related art, the quenching is performed at a temperature of 30° C. or less.

The primary aging is performed at a temperature ranging from 165° C. to 195° C. for 4 to 48 hours. As a preferable example, the drawing illustrates that the primary aging is performed at a temperature of 180° C. for 6 hours.

Unlike the related art, the present disclosure further includes the secondary aging. That is, the secondary aging is performed at a temperature ranging from 200° C. to 225° C. for 3 to 7 hours. As a preferable example, the drawing illustrates that the secondary aging is performed at a temperature of 215° C. for 6 hours.

In the AC4CH—Zr alloy of the present disclosure, an Al₃Zr precipitation-strengthening phase is formed during the aging. Since the precipitation phase is effectively formed at a temperature of 200° C. or more that is higher than the existing Mg₂Si precipitation-strengthening phase, additional aging is performed on the AC4CH—Zr alloy.

Meanwhile, a fine acicular Al₃Zr crystallization phase is formed even in the composition of Zr in an amount of 0.3 wt % or less, which causes deterioration in physical properties. In order to prevent this physical property deterioration, the quenching is performed at a temperature of 30° C. or less after the solutionizing in the present disclosure.

Tables 3 and 4 summarize a test result for physical properties according to the amount of Zr in the alloy composition of the present disclosure.

TABLE 3 Solutionizing: 535° C./6 hrs + Aging: 185° C./6 hrs AC4CH +Zr 0.007 +Zr 0.02 +Zr 0.05 +Zr 0.1 +Zr 0.15 Alloy Composition Alloy wt % wt % wt % wt % wt % Hardness (HB) 94.5 92.5 90 90.5 86 84 Elongation (%) 5.5 4.14 4.11 4.6 8.8 8.31 Yield Strength (MPa) 236 222 229 235 210 210 Tensile Strength (MPa) 293 284 289 289 272 271

TABLE 4 Solutionizing: 535° C./6 hrs + Aging: 170° C./6 hrs Alloy Composition AC4CH +Zr 0.015 +Zr 0.3 +Zr 1.0 Alloy wt % wt % wt % Hardness (HB) 99 99 99 94 Elongation (%) 3.11 6.46 6.13 0.68 Yield Strength (MPa) 244 248 249 232 Tensile Strength (MPa) 288 302 300 240

As indicated in Table 3, when 0.1 wt % or more of Zr is added to the alloy, the elongation of the alloy significantly increases. However, it can be seen that the alloy is decreased in hardness, yield strength, and tensile strength at room temperature, in inverse proportion to elongation.

In addition, when the aging condition is changed to prevent the deterioration of physical properties at room temperature, the alloy shows that an increment in elongation is decreased but strength properties are increased.

However, when the Zr content of the alloy exceeds 0.3 wt %, a coarse acicular Zr-related crystallization phase begins to appear as illustrated in FIG. 3, which is adversely affects the physical properties of the alloy. Therefore, the Zr content is preferably from 0.1 to 0.3 wt %.

In the heat treatment application of FIG. 2 according to the present disclosure, a physical property evaluation result for each Zr component is indicated in the following Table 5.

TABLE 5 +Zr 0.05 +Zr 0.1 +Zr 0.15 +Zr 0.3 +Zr 0.5 +Zr 1.0 Alloy Composition AC4CH wt % wt % wt % wt % wt % wt % Elongation (%) 6.3 6.7 8.6 9.2 8.8 3.3 0.68 Tensile Strength 273 269 268 270 267 254 190 (MPa) at Room Temperature Tensile Strength 125 128 141 144 145 121 115 (MPa) at High Temperature (250° C.)

A quenching temperature will be described below in more detail.

In the heat treatment process of the conventional alloy for cylinder heads, the quenching temperature is essential to be maintained at 80° C. or more. Since the cylinder head has a complicated internal structure, residual stress is apt to occur according to the cooling rate for each part in the cylinder head. In fact, the cylinder head may be damaged by the residual stress at the time of development. Therefore, as can be seen in FIG. 4, the quenching condition should be changed from water cooling to air cooling.

However, when the alloy of the present disclosure is quenched at a temperature of 80° C. or more, a fine acicular Zr phase is crystallized as illustrated in FIG. 5A. In this case, when 0.3 wt % or more Zr is added to the alloy, the effect of improving the physical properties of the alloy is inferior though the physical properties of the alloy are not considerably decreased compared to when the coarse crystallization phase appears.

However, when the quenching is performed at a temperature of 30° C. or less as in the heat treatment method of the present disclosure, the Zr-related crystallization phase becomes fine and is changed in shape from acicular to spherical as illustrated in FIG. 5B, so that the physical properties of the alloy can be improved.

A change in physical properties according to the change of Zr crystallization phase may be referred to in the following Table 6.

TABLE 6 Shape of Crystal- Yield Strength at Tensile Strength at lization Phase Room Temperature Room Temperature Elongation Acicular 244 289 4.4 Spherical 247 301 6.2

The term “crystallization phase” refers to a phase in which particles, which have not dissolved beyond the solid solution limit when molten Al is cooled, remain in the Al matrix. Generally, the crystallization phase may be observed when a material structure is analyzed using an optical microscope and an SEM. As a result of the Jmatpro thermodynamic simulation of FIGS. 6A and 6B, when Zr is added in an amount of 0.05 wt %, the solid solution of Zr reaches a threshold at a temperature of 476° C. or less so that a Zr-related phase begins to be crystallized. That is, when Zr is added in an amount of 0.05 wt %, the Zr-related phase is not crystallized at 650° C. or more, which is a temperature at which casting is generally performed. On the other hand, when Zr is added in an amount of 0.25 wt %, the solid solution of Zr reaches a threshold at 649° C. so that there is a sufficient possibility that a crystallization phase remains during cooling. This crystallization phase has an effect of strengthening materials by physically obstructing potential movement, but it rather weakens the physical properties of materials by acting as a stress concentration point when an acicular crystal is formed.

The term “precipitation phase” refers to a phase in which particles dissolved in molten Al exist in a supersaturated solid solution state due to quenching and are later precipitated into a solid by heat treatment. The crystallization phase is formed during solidification whereas the precipitation phase is formed during heat treatment (solutionizing+aging). Since the precipitation phase is fine enough not to be seen in general structure observation, it can be seen through TEM observation as illustrated in FIG. 7.

Furthermore, in the present disclosure, the secondary aging is performed after the quenching is performed at 30° C. or less and the primary aging is then performed.

If only the primary aging is conventionally performed in spite of the optimal composition of Zr, it can be seen that the treatment may not satisfy desired physical properties, namely all of high elongation, high thermal conductivity, and high-temperature durability.

This is due to the crystallization of acicular Zr structures and the absence of Zr-related precipitation-strengthening phases. The Mg₂Si precipitation phase, which is a strengthening phase of the existing AC4CH alloy, shows an optimum precipitation-strengthening effect when aging is performed at 180° C. for 6 hours, whereas Al₃Zr, which is a Zr-related precipitation phase, is precipitated into a matrix at a higher temperature to exhibit a strengthening effect. Accordingly, after the Mg₂Si precipitation phase is distributed in the matrix by performing aging at 160 to 195° C. for 4 to 24 hours, the secondary aging is further performed. Since the Mg₂Si precipitation phase begins to lose a strengthening phase effect when exposed to a high temperature of 200° C. or more, additional aging temperature and time are required to obtain the Al₃Zr precipitation phase, which is a high-temperature strengthening phase, while minimizing the effect degradation of the Mg₂Si precipitation phase.

The effects of strengthening phases are tested through physical property measurement, and the following Table 7 summarizes a result of the test. Table 7 indicates results when the secondary aging is performed at 200 to 225° C. for 3 to 7 hours under the conditions that the solutionizing is performed at 535° C. for 6 hours, the quenching is performed at 30° C., the primary aging is performed at 180° C. for 6 hours. In particular, as the result of additional aging at 215° C. for 6 hours, it can be seen that the strength properties at room temperature are slightly decreased due to the loss of Mg₂Si strengthening effect, but the strength properties at high temperature are improved due to the effect of Al₃Zr acting as a high-temperature strengthening phase.

In addition, performing the secondary aging can resolve the residual stress caused by the quenching at room temperature and obtain high elongation and high thermal conductivity by strength decreased at room temperature.

TABLE 7 Yield Tensile Yield Tensile Secondary Strength Strength Strength Strength Aging Secondary at Room at Room at High at High Thermal Temperature Aging Hardness Temperature Temperature Temperature Temperature Elongation Conductivity (° C.) Time (h) (HB) (MPa) (MPa) (MPa) (MPa) (%) (W/m · K) 0 0  99~102 247 301 116 125 6.2 155 180 6 100  247 296 111 120 6.1 154 180 12 96 244 297 117 122 6.6 156 180 24 92 236 276 119 127 6.8 157 215 3 81~87 229 275 126 141 8.4 167 215 6 79~86 229 272 128 146 9.3 169 245 6 55~65 110 149 97 109 12 159

As described above, it can be seen that the alloy of the present disclosure including Zr in the range has a tensile strength of about 272 MPa at room temperature and a tensile strength of about 146 MPa at high temperature as illustrated in Table 7. However, it can be seen that the alloy does not have desired physical properties when each heat treatment condition is not satisfied as illustrated in the following Table 8. Therefore, it is possible to derive optimum heat treatment conditions as illustrated in Table 9.

TABLE 8 Condition of Process Process Dissatisfaction Result (unit: MPa) Remark Solutionizing Temperature Min Tensile Strength at Room Process Value Temperature: 189 Temperature: 505° C. Max Part Shape/Dimension Process Value Damage Temperature: 570° C. Time Min Tensile Strength at Room Time: 3 hrs Value Temperature: 194 Max Part Shape/Dimension Time: 50 hrs Value Damage Quenching More than 30° C. Deterioration of Physical See Table 6 Properties by Acicular Zr Crystallization Phase Primary Temperature Min Tensile Strength at Room Process Aging Value Temperature: 217 Temperature: 155° C. Max Tensile Strength at Room Process Value Temperature: 166 Temperature: 200° C. Time Min Tensile Strength at Room Time: 3.5 hrs Value Temperature: 216 Max Tensile Strength at Room Time: 60 hrs Value Temperature: 223 Secondary Temperature Min Tensile Strength at Room Process Aging Value Temperature: 277 Temperature: 195° C Expected Residual Stress Occurrence Max Tensile Strength at Room Process Value Temperature: 149 Temperature: 245° C. Time Min Tensile Strength at Room Time: less than 3 Value Temperature: 275 hrs Expected Residual Stress Occurrence Max Tensile Strength at Room Time: 8 hrs Value Temperature: 164

TABLE 9 Temperature (° C.) Time (h) Critical Significance of Process Min Max Min Max Heat Treatment Condition Solution- 520 560 4 48 Dissolved into Reinforcing izing Element Al as Base Material Quenching — 30 — — Spheroidizing of Al₃Zr Crystallization Phase Primary 165 195 4 24 Formation of Mg₂Si Aging Precipitation Phase Secondary 200 225 3  7 Formation of Al₃Zr Aging Precipitation Phase

As described above, the alloy according to the present disclosure can exhibit the following effects as illustrated in the following Table 10.

The alloy is increased by 50% in elongation and 10% in thermal conductivity at room temperature, compared to existing materials. Although the alloy is somewhat decreased in strength properties (hardness: HB93→HB 78) at room temperature, it is increased in strength properties at the high temperature at which the engine is driven, for example, by 11% in yield strength and 17% in tensile strength, compared to existing materials. Overall, the alloy is increased by 30% or more in fatigue strength at a high temperature due to the increase in elongation, yield strength, and tensile strength. In addition, when the thermal conductivity of the cylinder head is increased by 10%, the torque of the engine is increased to result in an improvement in performance, which also helps to reduce knocking and thus improve fuel efficiency.

Furthermore, the current process in which quenching is performed at a temperature of 80° C. or more to reduce residual stress incurs the cost of maintaining water at a high temperature and the cost of excessive evaporation of water, whereas the present disclosure is expected to achieve a reduction in cost since quenching is performed at a temperature of 30° C. or less, compared to the related art.

TABLE 10 Alloy of Alloy of Present Related Art Disclosure Alloy AC4CH Added Element None 0.2 wt % of Zr Heat Treatment Solutionizing: 535° C./ Solutionizing: 535° C./ 6 hrs + Aging: 180° C./ 6 hrs + Aging: 180° C./ 6 hrs 6 hrs + Aging: 215° C./ 6 hrs Hardness at Room HB 93 HB 78 Temperature Yield Strength 115 MPa 128 MPa (11%↑) (@ 250° C.) Tensile Strength 125 MPa 146 MPa (17%↑) (@ 250° C.) Elongation (Room 5.5% 9.30% (50%↑) Temperature) Thermal 154 W/m · K 169 W/m · K (10%↑) Conductivity

In accordance with exemplary embodiments of the present disclosure, the aluminum alloy can be applied to the cylinder head of the high-power engine by having excellent high-temperature physical properties and high thermal conductivity in favor of performance and fuel efficiency.

More specifically, the aluminum alloy is increased by 50% in elongation and 10% in thermal conductivity at room temperature, compared to existing materials. Although the aluminum alloy is somewhat decreased in strength properties (hardness: HB93→HB 78) at room temperature, it is increased in strength properties at the high temperature at which the engine is driven, for example, by 11% in yield strength and 17% in tensile strength, compared to existing materials. Overall, the aluminum alloy is increased by 30% or more in fatigue strength at a high temperature due to the increase in elongation, yield strength, and tensile strength. In addition, when the thermal conductivity of the cylinder head is increased by 10%, the torque of the engine is increased to result in an improvement in performance, which also helps to reduce knocking and thus improve fuel efficiency.

Furthermore, the current process in which quenching is performed at a temperature of 80° C. or more to reduce residual stress incurs the cost of maintaining water at a high temperature and the cost of excessive evaporation of water, whereas the present disclosure is expected to achieve a reduction in cost since quenching is performed at a temperature of 30° C. or less, compared to the related art.

While the specific embodiments have been described with reference to the drawings, the present disclosure is not limited thereto. It will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the disclosure as defined in the following claims. Therefore, these changes and modifications will fall within the scope of the disclosure as long as they are apparent to those skilled in the art, and the scope of the present disclosure should be defined based on the entire content set forth in the appended claims. 

What is claimed is:
 1. A method of manufacturing aluminum alloy, comprising: solutionizing a cast product made of an aluminum alloy containing 0.1 to 0.3 wt % of Zr; quenching the cast product at a temperature of 30° C. or less after the solutionizing; and aging the cast product after the quenching.
 2. The method of claim 1, wherein the solutionizing is performed at a temperature ranging from 520° C. to 560° C. for 4 to 48 hours.
 3. The method of claim 1, wherein the aging comprises primary aging and secondary aging after the primary aging so as to be performed twice.
 4. The method of claim 3, wherein the primary aging is performed at a temperature ranging from 165° C. to 195° C. for 4 to 48 hours.
 5. The method of claim 4, wherein the secondary aging is performed at a temperature ranging from 200° C. to 225° C. for 3 to 7 hours.
 6. A method of manufacturing aluminum alloy, comprising: solutionizing a cast product made of an aluminum alloy containing 0.1 to 0.3 wt % of Zr; quenching the cast product after the solutionizing; and aging the cast product after the quenching, wherein the aging comprises primary aging and secondary aging after the primary aging so as to be performed twice.
 7. The method of claim 6, wherein the primary aging is performed at a temperature ranging from 165° C. to 195° C. for 4 to 48 hours.
 8. The method of claim 7, wherein the secondary aging is performed at a temperature ranging from 200° C. to 225° C. for 3 to 7 hours.
 9. An aluminum alloy comprising: Al as a base material; 6.5 to 7.5 wt % of Si; 0.35 to 0.45 wt % of Mg; and 0.1 to 0.3 wt % of Zr.
 10. The aluminum alloy of claim 9, further comprising 0.2 wt % or less of Cu.
 11. The aluminum alloy of claim 9, wherein an Al₃Zr precipitation-strengthening phase is formed in the aluminum alloy. 